Starting a brushless DC motor

ABSTRACT

A brushless DC motor is started by initially specifying an arbitrary rotor position and applying such drive currents to the three motor phase inputs that the rotor moves to the predetermined position and thereafter applying a current pulse to the appropriate winding to cause the rotor to move in the desired rotational direction. After the motor has begun to turn, the current supply to the motor phase inputs is interrupted. The current in the motor is allowed to decay to a level where the I and R voltage drops in the windings are substantially less than the back EMFs at the speed at which the motor is turning. The small back EMFs generated in the windings are sampled and monitored to detect whether the rotor has moved to a position requiring a change in commutation. Drive current is supplied to the three motor phase inputs in accordance with whether or not a need for a change in commutation was detected by the sample so that the rotor will continue to turn in the required direction. The current interruption and sampling are repeated at a rate greater then the commutation rate (i.e. more frequently as the motor speed increases) and at a rate directly proportional to the speed of the motor to determine the current position of the rotor from the sampled back EMFs. When the back EMFs are significantly greater than the IR losses in the windings, current is supplied to the motor phase inputs (i.e. commutate) in dependence on the current position of the rotor as determined by the continuous comparison of the back EMFs.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a method of starting a brushless directcurrent motor and to a drive arrangement including such a motor.

BACKGROUND OF THE INVENTION

In the past, most brushless DC motors used dedicated sensors such asHall effect devices or optical sensors to determine the angular positionof the rotor with respect to the stator. This information was then usedto commutate the motor. More recently, the back EMF signals from thestator coils of the motor have been used to sense the angular positionof the rotor with the advantages of reducing commutation timing errorsand costs. However, when the motor is stationary there is no back EMFfrom the stator coils and so a special start system is required. Thissystem is also required at low speeds as the drive currents causevoltages that can be much greater than the back EMF signals making theirdetection a problem.

Design Engineering October 1984, page 37 discloses a back EMF positionsensing system for brushless DC motors. This article is not clear as tothe details of the starting system used but describes a technique whichonly allows energisation of the motor coils during particular timeperiods which is incompatible with many applications.

EP-A-251785 discloses a method of producing feedback informationconcerning the rotational position of the rotor of a brushless dc motorwithout using sensing devices in the structure of the motor, when themotor is at a standstill or rotating only slowly. The phases of themotor are energised under the control of a microprocessor and currentthrough the phases is monitored by the microprocessor. Periodically,from standstill to a first rotational speed, each of the set of motorphases is momentarily energised in sequence and the amplitude of theshort current pulse that flows in each phase is monitored. The phase inwhich the highest current pulse flows indicates the position of therotor. The momentary energisation of the set of phases is insufficientto generate sufficient torque to turn the rotor: each momentaryenergisation of the set of phases is followed by sustainedtorque-producing energisation of the particular phase which isappropriate to the rotor position indicated by the monitored currentpulses which flowed in response to the momentary energisation.Monitoring the amplitudes of the short current pulses requires the useof an analogue-to-digital converter. Accordingly, the method disclosedin EP-A-251785 has the disadvantage of either restricting the choice ofthe microprocessor to one with an integral analogue-to-digital converteror requiring the use of a separate analogue-to-digital converter whichwould be accompanied by extra cost, power consumption and lower systemreliability.

DE-C2-32 09 394 discloses a system of controlling a brushless DC motorin order to get the motor turning in the desired direction as soon aspossible. With the motor at a standstill, a current pulse is applied toone of the windings and then the back EMFs in the windings are examinedto determine whether the motor has begun to turn in the desireddirection. If it has, the system runs the motor in a self-commutatingmode. If the motor has begun to turn in the undesired direction, thesystem causes the windings to be energised so that the motor changes itsdirection of rotation. The system disclosed in this prior document issuitable where a load is not applied until the motor has reached itsoperating speed but is not applicable to an arrangement where the motorhas a high starting load, such as in a record disk file drive.

The invention seeks to provide an improved method of starting abrushless direct current motor, particularly one having a high startingload such as one driving a record disk file.

DISCLOSURE OF THE INVENTION

The invention provides a method of starting a brushless direct currentmotor including the successive steps of (a) energising the motorwindings to move the rotor to a predetermined position; (b) applying adrive current pulse to the appropriate winding phase to cause the rotorto turn from the predetermined position in a predetermined direction ofrotation; (c) sampling the back EMFs generated in the winding phaseswhen the current pulse has decayed to a level where the I×R voltagedrops in the windings are substantially less than the back EMFs in orderto indicate whether a change in commutation is required to keep therotor turning in the predetermined direction; and (d) applying a drivecurrent pulse to the appropriate winding phase in response to theindication provided by step (c) to keep the rotor turning in thepredetermined direction.

The invention also provides a drive arrangement including a brushlessdirect current motor; means to energise the motor windings to move therotor to a predetermined position; means to apply a drive current pulseto the appropriate winding phase to cause the rotor to turn from thepredetermined position in a predetermined direction of rotation;sampling means to sample the back EMFs generated in the winding phaseswhen the current pulse has decayed to a level where the I×R voltagedrops in the windings are substantially less than the back EMFs in orderto indicate whether a change in commucation is required to keep therotor turning in the predetermined direction; and means to apply a drivecurrent pulse to the appropriate winding phase in response to theindication provided by the sampling means to keep the rotor turning inthe predetermined direction.

BRIEF DESCRIPTION OF THE DRAWING

How the invention can be carried out will now be described by way ofexample, with reference to the accompanying drawing which is a diagramof a brushless DC motor and apparatus for controlling its start-up andrunning.

DETAILED DESCRIPTION OF THE INVENTION

An electronically commutated DC motor M has an 8 pole permanent magnetrotor R made of a rare earth magnetic material, such asneodymium/iron/boron. The stator windings 11, 12, 13 of the motor M areconnected in the delta configuration to provide three phases X, Y and Zwhich are energised from a voltage supply +V by electronic switchingcircuitry 14. A controller 15 controls the commutation and speed of themotor M by supplying signals on six output lines 01 to 06 to theswitching circuitry 14. Rotation of rotor R of the motor M induces backEMFs in the windings 11, 12, 13 and hence in the phases X, Y and Z. Thephases X, Y and Z are connected to low pass filter circuitry 16 toseparate out the back EMFs. The time constant of the low pass filters inthe circuitry 16 should be large enough to filter out switching noisefrom the motor drive circuits and should be chosen such that the phaseA, B and C (with respect to the rotor position) are correct at therequired operating speed of the motor. Appropriate pairs of theseparated out back EMFs are compared by three comparators 17, 18 and 19to produce outputs A, B and C which together provide an indication ofthe position of the rotor R all the time that the back EMFs are greaterthan the I×R voltage drops in the windings. The comparators 17, 18 and19 must be such that they function with inputs down to zero volts, andthey can be comprised in a standard quad comparator package, such as theLM 339 package marketed by National Semiconductor Corporation. When themotor is running at a speed greater than a predetermined minimum speed,the switching means 20 is set as shown in the drawing and the outputs A,B and C are passed onto inputs N1, N2 and N3 of the controller 15. Inthis case the controller 15 generates signals on output lines 01 to 06in accordance with the rotor position indicated by the outputs A, B andC in an analogous manner to that in which commutation signals areproduced by the commutation logic conventionally used with motorsemploying three Hall effect sensors to determine the angular position ofthe rotor.

The outputs A, B and C are also supplied as inputs to a 3-bit register21 which samples them on receipt of a load pulse LP and produces outputsA1, B1 and C1 which together represent the position of the rotor at thesampling instant defined by load pulse LP. The outputs A1, B1 and C1 areapplied to a digital signal processor (DSP) 22 which generates therefromoutput signals A2, B2 and C2 which are such as to control the driving ofthe motor in the required direction from the sampled rotor position. Theoutput signals A2, B2 and C2 are applied to inputs N1, N2 and N3 of thecontroller 15 when the switching means 20 is set in the oppositeposition to that shown in the drawing. The position of switching means20 is determined by a signal S generated by the DSP 22. The load pulseLP applied to the 3-bit register 21 can either be generated by the DSP22 or it can be provided by a pulse train having a predetermined period.The DSP 22 also generates a disable current signal D which when appliedto the controller 15 prevents the switching circuitry 14 from supplyingany current to the windings X, Y and Z. The switching circuitry 14comprises three P-type FETs 23, 24 and 25 and three N-type FETs 26, 27and 28. FETs 23 and 26 are connected in series between the voltagesupply +V and ground and the junction between FETs 23 and 26 isconnected to the X phase input of the motor M. The output line 01 of thecontroller 15 is connected to the gate of FET 23 and the output line 04to the gate of FET 26. In a similar manner, the FETs 24 and 27 areconnected in series between the voltage supply +V and ground, and to theY phase input of the motor and to the output lines 02 and 05 of thecontroller 15. The FETs 25 and 28 are similarly connected to the Z phaseinput of the motor and the outputs 03 and 06 of the controller 15.

To start the motor M, the DSP 22 supplies the signal S continuously tothe switching means 20 to set the switches therein to the oppositepositions to those shown in the drawing and supplies signals on theoutputs A2, B2 and C2 which are such that the controller 15 respondsthereto to cause the motor phases to be energised in an initialpredetermined sequence to cause the rotor to move from its rest positionto an arbitary predetermined position. The current supplied to the motorphases is steadily increased during this initial period of energisationin order to prevent overshoot. When the rotor has assumed thepredetermined position, the DSP 22 causes the controller 15 to apply acurrent pulse to the motor phases in such a sense and sufficiently longthat the rotor will turn in the predetermined direction from itspredetermined position. After a time interval sufficiently long that therotor has begun to turn in response to energisation of the motor phases,the DSP 22 supplies the disable signal D to the controller 15 causingthe currents in the motor windings to decay. After a time sufficientlylong that the currents in the motor windings have decayed substantiallyto zero, the load pulse LP is applied to the 3-bit register 21 whichthen samples the outputs A, B and C of the comparators 17, 18 and 19.The sampled outputs A1, B1 and C1 are passed onto the DSP 22 whichdetermines therefrom the position of the rotor and generates outputsignals A2, B2 and C2 which will make the rotor turn in the desireddirection. Simultaneously, the DSP 22 turns off the disable signal Dthus allowing the controller 15 to respond to the signals A2, B2 and C2and to supply signals on its outputs 01 to 06 such as to cause theappropriate phase of the motor to be energised to turn the rotor in thedesired direction. After an interval, the DSP 22 again supplies thedisable signal D to the controller 15, thus causing the currents in themotor windings to decay. When they have decayed to a level such that theback EMFs are greater than the I×R losses, the load pulse LP is againapplied to the 3-bit register 21 to sample the outputs A, B and C of thecomparators. The sampled outputs are passed onto the DSP 22 and onceagain it turns off the disable signal D allowing the controller 15 toenergise the appropriate phase of the motor. This process is repeated ata rate directly proportional to the speed of the rotor until the backEMFs are significantly greater than the I×R losses in the windings(without the need to disable their energisation). During this period,the back EMFs are sampled at a rate greater than the commutation rate,the commutation rate being continuously monitored by the DSP 22.

Periodically, for example after every sixth sampling of the back EMFs,the DSP 22 determines the actual position of the rotor from the sampledback EMFs. On receipt of the other samplings of the back EMFs, the DSP22 simply determines whether a change in rotor position requiring achange in commutation has occurred. If such a change in rotor positionhas occurred, the DSP 22 causes the application of the next commutationphase that is appropriate for torque generation in the desired directionof rotation. This ensures that the motor will only start to run in theintended direction.

When the rotor is turning at a speed such that the back EMFs aresignificantly greater than the IR losses in the windings, the DSP 22cuts off the signal S, thus setting the switches in the switchingcircuitry 20 to the positions shown in the drawing. Thereafter, theoutputs A, B and C of the comparators 17, 18 and 19 are passed to theinputs N1, N2 and N3 of the controller 15 and the disable signal Dremains off while the motor continues to run. Under the control of thecontroller 15, the speed of the motor is built up to the desired speedand then the desired speed is maintained.

Although the invention has been described in connection with the motorhaving its windings connected in the delta configuration, it could beapplied to a motor having star-connected windings.

To summarise, there has been described herein a method of starting abrushless dc motor which comprises the following basic steps:

1. Specify an arbitary rotor position and apply such drive currents tothe three motor phase inputs that the rotor moves to the arbitary rotorposition.

2. Apply a current pulse to the appropriate winding such that the rotormoves in the desired direction of rotation.

3. After the motor has begun to turn, stop the current supply to themotor phase inputs.

4. Allow the current in the motor to decay to a level where the I×Rvoltage drops in the windings are substantially less than the back EMFsat the speed the motor is turning at.

5. Sample the (very small) back EMFs generated in the windings.

6. Monitor the sampled back EMFs to detect whether the rotor has movedto a position requiring a change in commutation.

7. Supply drive current to the three motor phase inputs in accordancewith whether or not a need for a change in commutation was detected instep 5 and so that the rotor will continue to turn in the requireddirection.

8. Repeat steps 4 to 7 a predetermined number of times at a rate greaterthan the commutation rate (i.e. more frequently as the motor speedincreases) and at a rate directly proportional to the speed of themotor.

9. Repeat steps 4 and 5.

10. Determine the current position of the rotor from the sampled backEMFs.

11. Supply such current to the three motor phase inputs that will causethe motor to turn in the required direction from the current position.

12. Repeat steps 4 to 11 until the back EMF voltages are significantlygreater than the IR losses in the windings.

13. When the back EMFs are significantly greater than the IR losses inthe windings, supply current to the motor phase inputs (i.e. commutate)in dependence on the current position of the rotor as determined bycontinuous comparison of the back EMFs.

I claim:
 1. A method of starting a brushless direct current motor (M), including the successive steps of(a) energising the motor windings (X, Y, Z) to move the rotor to a predetermined position; (b) applying a drive current pulse to the appropriate winding phase to cause the rotor to turn from the predetermined position in a predetermined direction of rotation; (c) sampling the back EMFs generated in the motor windings (X, Y, Z) when the current pulse has decayed to a level where the I×R voltage drops in the motor windings are substantially less than the back EMFs in order to indicate whether a change in commutation is required to keep the rotor turning in the predetermined direction; and (d) applying a drive current pulse to the appropriate winding phase in response to the indication provided by step (c) to keep the rotor turning in the predetermined direction.
 2. A method as claimed in claim 1, further including the steps of:(e) determining the rotor position from the sampled back EMFs; and (f) applying a drive current pulse to the appropriate winding phase to turn the rotor in the predetermined direction from the rotor position determined in step (e).
 3. A method as claimed in claim 2, further comprising repeating steps (e) to (f) until the motor reached a speed at which the back EMFs are significantly greater than the IR losses in the windings and then commutating the motor in dependence on the current position of the rotor as indicated by continuous comparison of the back EMFs generated in the windings.
 4. A drive arrangement including a brushless direct current motor (M);means to energise the motor windings (X, Y, Z) to move the rotor to a predetermined position; means to apply a drive current pulse to the appropriate winding phase to cause the rotor to turn from the predetermined position in a predetermined direction of rotation; sampling means to sample the back EMFs generated in the motor windings (X, Y, Z) when the current pulse has decayed to a level where the I×R voltage drops in the windings are substantially less than the back EMFs in order to indicate whether a change in commutation is required to keep the rotor turning in the predetermined direction; and means to apply a drive current pulse to the appropriate winding phase in response to the indication provided by the sampling means to keep the rotor turning in the predetermined direction. 